This invention relates to brushless dynamoelectric machines and, more particularly, to such machines having rotatable control wheel assemblies for the excitation control circuit.
Reference is made to Heyne et al. U.S. Pat. No. 4,456,843, June 26, 1984, for background to the present invention. The patent describes a brushless dynamoelectric machine provided with a control wheel of molded insulating material. The wheel has a rim on the inner surface of which the rectifiers and other components of the starting control circuitry are directly mounted. In a specific embodiment of the patent, starting, or discharge, resistors are provided of wound wire on the outer surface of the control wheel rim with provision for their mechanical support and ventilation. During the further development of the molded control wheel assembly, improvements have been made which are now presented in this patent application.
The objectives sought are to reduce cost and weight and to provide an insulated control wheel on which the components are arranged with a high power density; that is, in a very compact unit. The design of the prior patent did so to a degree, but the arrangement of the present invention does so even more effectively and is particularly more suitable for machines with shaft speeds of 1800 rpm or more.
By the present invention, the molded insulating control wheel concept of the prior patent is retained, in that excitation control components are mounted on the inside of a rim portion of a structure of molded insulation material. However, instead of using a wire-wound resistor on the rim exterior, a centrally located resistor wheel is used. The resistor wheel is a metal cylinder located radially within the rim of the control wheel and proximate to the hub of the control wheel where it is secured. A major portion of the resistor wheel is of solid metal having a plurality of axially extending apertures, in each of which is located a resistance element, all of which are interconnected to form a field discharge resistor. The resistor elements are each preferably a cartridge heater of a resistance wire within a metal jacket and having a solid mass of thermally conductive, electrically insulative, material between the wire and the jacket. The mounting of the resistance elements is such that the metal jackets fit closely against the adjacent metal of the resistor wheel for a substantially direct heat transfer therebetween. It is therefore seen that among the key changes from the prior patented design to the presently preferred design are that the resistors are now centrally located rather than on the outside of the rim, the resistors are cartridge resistors instead of wire-wound resistors, and the resistors are enclosed in a substantially metallic structure for heat dissipation rather than relying on ventilation.
Additionally, the resistor wheel, in its location directly radially inside components mounted on the inner rim of the control wheel, is provided with longitudinally or axially extending fins or blades. The fins or blades help to dissipate heat from the resistors and also serve as fan blades directing air against the components on the control wheel rim for overall improvement in the power density of the arrangement.
The components on the inner surface of the wheel rim include finned heat sinks for the diode rectifiers and SCRs of the control circuit. For further improvement in heat transfer, the fins of the heat sinks are made to extend at an angle of up to about 45.degree., such as about 25.degree., to the direction of rotation.
The system as described above and more particularly described in the following description of preferred embodiments has made possible the provision of an integrated control wheel and starting resistor package which in contrast to formerly conventional designs such as that illustrated in FIG. 2 of the above-mentioned patent can yield a weight reduction of about 45-70% and an inertia reduction of about 75 to 90%. This will allow operation on high speed machines (at least 1800 rpm) with the control package mounted outside of the bearing adjacent the motor in an overhung arrangement with minimized shaft stresses caused by lateral and torsional excitation forces.